The art of obtaining cores of soft formations or loosely consolidated and unconsolidated earth materials, hereinafter collectively referred to as "unconsolidated material," requires apparatus and techniques different from the obtaining of cores in hard rock. Many features of core sampling apparatus for unconsolidated materials and for hard rock formations are similar. However, the drilling techniques cannot be the same. In obtaining a core from a hard rock formation, the drill bit must be lubricated and cooled and the cuttings scavenged by fluid flow as the bit cuts the earth about a core. The bit will lead and undercut the core barrel to permit the core to move upwardly and into the barrel for subsequent recovery. In obtaining a core of unconsolidated material, a fluid cannot ordinarily be used since it can easily alter or break apart the core structure. Moreover, the core barrel must lead the cutting bit which circumscribes the core barrel and push its way into the formation ahead of the bit, to maintain the integrity of the core.
It is often necessary to core drill and obtain cores of unconsolidated materials at construction projects and the like, as where the nature of the overburden above bedrock is to be ascertained. Ordinarily, the core drilling of unconsolidated materials will not exceed a depth of more than several hundred feet. Accordingly, a specialized drilling apparatus has been developed for this purpose, which uses a hollow auger with a hollow bit for drilling and removing material from the hole as it is being drilled. A big advantage is that a drilling fluid is not necessary. A hollow auger may have an outside diameter of 8 inches, more or less, with the auger flights mounted upon a hollow axial tube having an inside diameter of 4 inches, more or less.
In drilling operations, sections of the auger, as of five, ten or twenty foot lengths, are connected together to extend the drill string as the depth of the hole increases. In one system of operation, sectional stems having lengths corresponding with the sections of the drill string are extended into the sleeve to carry a drill bit to cut away the central portion of the hole when coring is not desired. With present practice, when cores are to be recovered, the stems are removed and a lightweight core barrel, of a relatively short length, is lowered into the auger sleeve to the bottom of the hole and then driven into the earth ahead of the drill, by a weight raised and released to produce a drop hammer effect. When the core barrel is driven as far as possible, usually from 12 to 18 inches, the hollow auger is rotated to move downwardly to the base of the core barrel. The loaded core barrel is then pulled out of the stem to recover the core within it.
A common problem in such a coring operation resides in the fact that soft and unconsolidated materials will usually jam within the core barrel when the core barrel is driven into the earth ahead of the drill. Core samples having a length of one foot, two feet or three feet at the most, are all that can be expected in such an operation. Moreover, when the core barrel is driven into the earth, the material within the core barrel, i.e., the core specimen, may be compacted or otherwise distorted to such an extent that the core can give erroneous indications when tested for properties, such as compressibility, permeability and the like. Finally, the core is often jammed into the barrel so tightly that the barrel has to be split or cut longitudinally to remove the core. This requires thin wall, single use core barrels. Therefore, in the present state of the art, the criticism that compacted core samples are not much better than drill cuttings appears to have some justification.